Well accepted printing systems in an "office-environment" as e.g. ink-jet printers and electrostatographic printers, are not used as much as would be expected when the convenience of these systems is considered. Most of these printers can only partially print continuous tone images and the continuous tone image has to be specially treated (e.g. by a dither method) before the print can be made. In this context, a continuous tone image or contone image is an image containing grey levels, with no perceptible quantisation to them. This drawback has hampered the use of these very convenient printers in those imaging areas where it is important to accurately print continuous tone images as e.g. in pictorial photography, medical imagery, etc.
In an ink-jet printer, a convenient printing system for use in an office environment, it has been proposed in EP-A-0 606 022 to use different inks, with different pigmentation and to use the ink with low pigmentation to print the low densities and the ink with high pigmentation to print the high densities. In this technique use is made of ink drops with volumes ranging from 25 to 100 .mu.l in the so called bubble jet based systems, or with volumes in the range of 5 to 10 .mu.l in the so called continuous jet systems. In all cases the images are built up by combining in an appropriate way such drops on the substrate, and although the addressability of each drop typically lies in the range of 300 dpi (dots per inch) to 1200 dpi, the not fully reproducible way the dot spreads and penetrates in the substrate limits the real resolution in the printed image. Hereinafter the resolution of image will be described in dpi, a normal description in the printing business. Further attempts to reproduce continuous tone images using light- and dark-colored inks have been described in EP-A-0 606 022 and U.S. Pat. No. 4,860,026.
Electro(stato)graphic printers are evenly well accepted imaging systems in an "office environment" as ink-jet printing since these systems, e.g. electrophotographic copiers, electrographic printers, Direct Electrostatic Printing (DEP), are convenient, fast, clean and do not need aqueous solutions. Since electro(stato)graphic systems may use solid particles that typically have a particle diameter between 1 and 10 .mu.m as marking particle, it is possible to achieve very high resolution in electro(stato)graphy.
However, most electro(stato)graphical imaging systems, are not intrinsically capable of forming continuous tone and special measures have to be taken to print continuous tone images.
Continuous tone printing in electrophotographic printing by a laser beam is described in the Journal of Imaging Technol., Volume 12, n.degree. 6 December 1986 on pages 329 to 333 in an article entitled "Electrophotographic Color Printing Using Elliptical Laser Beam Scanning Method". In this article a dot matrix method, combined with pulse-width modulation of the laser beam (to be able to introduce in each dot of the matrix several density levels) and with an elliptical laser beam, is described to achieve a continuous tone reproduction with sufficient resolution and linearity over a tone range of 256 levels. Although with such a printing system quality continuous tone prints can be made, there are still some problems to be addressed. On an electrostatic photoreceptor there is a threshold level of toner adhesion : this means that in the low density areas, where the electrostatic latent image is weak and is situated just above that threshold, the system shows inherently some instability in the low density areas. Also, since the low density areas are printed using very few toner particles, the granularity (in other terms graininess or noise) in the low density areas becomes easily objectionable for high quality prints.
In Patent Abstract of Japan vol. 007 no. 290 (p. 245), Dec. 24, 1983 & JP-A-58 162970 (Hitachi Seisakusho KK), Sep. 27, 1983 a second toner having a same color and a lower color density (1.0 black density) is added to a first toner (1.8 black density) in a 4:1 ratio to obtain a good gradation.
In U.S. Pat. No. 5,142,337 a second toner is used, comprising a mixture of opaque black, opaque white and clear toner. A second toner layer is applied on top of a first toner layer, comprising black toner.
In proceedings of the International Congress on Advances in on-Impact Printing Technologies, San Diego, Nov. 12-17, 1985, no. Congress 5, Nov. 12, 1989, Moore J., pages 331-341, Kunio Yamada et al `Improvement of halftone dot reproducibility in laser-xerography`, the author discusses graininess of the xerographic process, mainly influenced by dot growth.
In EP-A-0 275 636 a cyan, magenta, yellow and black toner combination is disclosed for color printing applications.
In Journal of Imaging Technology, vol. 15, no. 5, October 1989, pages 198-202, Tanaka T `Color Reproduction in Electrophotography: a layered model`, Tanaka discloses a method predicting color from color toner weight and vice versa.
The intrinsic qualities of electro(stato)graphic printers (speed, resolution, cleanness, dry operationable) have not yet been used in instances where speed, cleanness and dry operationability are highly wanted, just because of the problems cited above. A particular, but not limiting, example of an area where electro(stato)graphic printing could advantageously be used, if good, stable, high resolution half-tone (continuous tone) printing over at least 256 printed (not only addressed) density levels were possible, is the medical hard-copy sector.
There is thus need for electro(stato)graphic systems being capable of printing continuous tone images.